In a prior art high-frequency inverter, the operating frequency has varied according to a type of the load (pan) to be heated or a change in an input power. When such a high-frequency inverter is applied to a multi-burner induction heating cooker having a plurality of heating burners, there has been the problem that a pan interference noise is generated due to a frequency difference among the burners. As a high-frequency inverter for solving this problem, there is one disclosed in U.S. Pat. No. 5,571,438 as shown in FIG. 71. In this figure, the high-frequency inverter is comprised of a DC power source 101, an inverter circuit 102 for converting a direct current from the DC power source 101 into a high-frequency current and a control circuit 103 for controlling this inverter circuit 102. The inverter circuit 102 is constructed of a reverse current blocking type first switching element 104, a reverse current conducting type second switching element 105, a coil 106 for heating use, a first resonance capacitor 107, a second resonance capacitor 108 and a diode 109. The control circuit 103 includes a drive section 110 which makes the first switching element 104 and the second switching element 105 alternately conductive at a constant frequency f0.
FIG. 72 shows operation waveforms at several portions of the high-frequency inverter constructed as above. As shown in FIG. 72, the drive circuit 110 drives the inverter circuit 102 by alternately turning on the first switching element during a time ton1 and the second switching element during a time ton2 in a constant cycle t0. As apparent from FIG. 72, a zero voltage switching operation is achieved in this high-frequency inverter. FIG. 73 shows an input power control characteristic of this high-frequency inverter. As shown in this figure, the electric power inputted to the high-frequency inverter is controlled by the conducting time ton1 of the first switching element. That is, in the high-frequency inverter, an input power P.sub.in is controlled by changing the ratio (ton1/t0) of the conducting time ton1 of the first switching element 104 relative to the constant cycle t0 at the constant operating frequency f0.
As described above, the prior art high-frequency inverter can execute the input power control at the constant operating frequency. Therefore, when it is applied to the multi-burner induction heating cooker, the pan interference noise attributed to the frequency difference between the burners can be solved. Also, the zero voltage switching operation can be achieved with the two switching elements, and this can achieve a reduced power loss and a reduced generated noise in the switching operation.
The multi-burner induction heating cooker is expected to be further popularized in future for its high convenience. In accordance with it, a further improvement is expected for the high-frequency inverter to be applied to the induction heating cooker, and a high-frequency inverter achieving further reductions in size and cost is desired.
The present invention has been developed for solving the aforementioned problems and provides a high-frequency inverter which operates at a constant frequency implemented by a simple circuit construction and an induction heating cooker to which the inverter is applied.